[SamReidHughes]

(a b c) is (a b c . nil). That is, (a . (b . (c . nil))). On the other hand, (a b . c) is (a . (b . c)). (Edit: rewrote post)

[imdhmd]

() is equivalent to an empty list and nil according to this notation, in which case both would be proper lists. But according to pg, in bel, (a b . c) is not a proper list.

Edit: I think I understand now, hmm.

[erik_seaberg]

Think of a proper list like a degenerate tree with all the values on the left and structure on the right. A dotted list puts the last value on the right, where nil would usually be.

Dotted lists are pretty rare. They mostly show up in association lists, where key/value pairs are stored as

  ((name . dave) (type . user))

and adding a pair to the front of the list shadows any other pairs with the same key.

[nilkn]

While I think this is the correct interpretation, I do not think it's actually implied by the language specification. In fact, I think the notation (a b . c) is simply undefined -- there's no way to formally deduce what it means from what is defined.

[SamReidHughes]

> 2. When the second half of a pair is a list, you can omit the dot before it and the parentheses around it. So (a . (b ...)) can be written as (a b ...).

That defines (a b . c). Edit: Note that (b . c) is a list.

[nilkn]

It doesn't. That definition applies only to expressions like (a b c) which do not contain the dot operator at all. (a b . c) is not of the form specified. Moreover, every object of the sort defined via (a b c ...) is a proper list, and (a b . c) is, as described in the specification, not a proper list, so it can't be covered by that definition. It's true that (b . c) is a list, but the form (b . c) does not occur in the expression (a b . c) so this isn't relevant. (You'd need the expression under analysis to instead be (a (b . c)).)

[SamReidHughes]

The rule I quoted states that (a . (b . c)) can be written as (a b . c).

[nilkn]

That would only be true if (b . c) were a proper list. But it’s not. Strictly speaking, it’s not even a list if you go by the definition of list provided before your quoted rule. It’s later called a “dotted list” to distinguish it from a list, which must be nil-terminated.

Your position boils down to claiming that the rule you quoted is intended to cover lists as well as dotted lists. As stated, it only covers the former, which leaves (a b . c) undefined.

[SamReidHughes]

In the usage seen later throughout the document, dotted lists are a kind of list. It doesn't say its examples of how to build a list are exhaustive.